It is the fin!
I've found a select SL9 33 and used today with the I-sonic 86 2009 and a NP Rs Slalom 6.2 and made a few runs at 31-32 knots (31.12 in 10 seconds) ... without any spin out.
The difference with the Drake fin is ... Huge! The board was faster, but better than that I felt myself really confident in pushing with the back feet without any problem or surprise.
Thanks everyone for the advices ... lots of thing to learn in order to trim the board and the sail and go faster ... but ... fins matters!